The system demand shown on this graph is the total instantaneous MW demand being supplied across England, Wales and Scotland. It is estimated from the generation data (Instantaneous Generation by Fuel Type) by summing the output of all the generation connected to the GB Transmission System (including Embedded Large Power Stations), plus imports from External Interconnectors e.g. from France-GB and Ireland-GB. Negative values for the Interconnector and Pumped-Storage and Non-Pumped-Storage Hydro types are set to 0 in the summation step.